Electronic devices which may be positioned on or close to the body of a user to enable biometric parameters to be monitored are known. For example, devices which may be used to measure heart rate, blood oxygenation levels, temperature, galvanic skin conductance and other biometric parameters are known. These devices may comprise light sources which may be used to illuminate a portion of skin of the user. The biometric parameters can then be determined by monitoring the amount of light absorbed or scattered by the illuminated portion of skin.
It is useful to provide electronic devices which can be used to monitor biometric parameters for an extended period of time. This requires an electronic device which can be worn by a user for the extended period of time without causing discomfort to the user. It also requires that the electronic device has low power requirements so that a reliable output signal can be provided over the extended period of time.